How easy is it to replace the battery for Gardtec 300 Series Alarm?

I have a Gardtec 300 Series alarm which I have had for about 9 years now. Recently it seems to go off regurlarly every week for no apparent reason. Sometimes it goes off when we enter the house as the main entrance door has door sensor attached. When there is a power shut down the alarms goes off for about 20 minutes non stop. Can I or can I not stop the alarm during a power shut down. I beleive the internal battery is dead and needs replacing. Is that right if so what and were exactly am I looking for in the alarm control panel box.